1. Field of the Invention
This invention relates generally to a circuit for controlling the orientation of magnetic fields. The circuit has particular application as a control circuit for a flux shunt trip device in a circuit breaker. The circuit counteracts the effects of external magnetic fields which could otherwise cause undesired nuisance tripping of the breaker.
2. Background Information and Description of the Prior Art
Circuit breakers with electronic trip units commonly employ a flux shunt trip device that incorporates permanent magnets. The flux shunt trip device has a springloaded plunger which when released to its extended position causes the circuit breaker to trip. Under normal operating conditions, the plunger is maintained in a retracted position by the force of the permanent magnet. A trip coil is wound in intimate contact with the plunger. When the trip coil is energized with current, a magnetic flux is thereby generated. This magnetic flux opposes the magnetic flux of the permanent magnet. When the opposing flux is of sufficient strength to counter-balance the magnetic flux of the permanent magnet, the spring force on the plunger then causes the plunger to move to its trip position.
Due to the restricted space available inside a circuit breaker, it is often necessary to mount the shunt trip adjacent to the phase conductor and associated current transformers. A problem has been encountered at high current levels because the current in the phase conductor and current transformers create a separate magnetic flux which can act to oppose the flux of the permanent magnet and thereby cause the shunt trip to trip the breaker prior to reaching the trip current value. This results in undesired nuisance tripping.
One solution to the nuisance trip problem is disclosed in U.S. Pat. No. 4,731,692. This patent requires two coils on the shunt trip device: an assist coil and a trip coil. The trip coil acts in the manner discussed hereinbefore. The assist coil acts to assist the action of the permanent magnet. More specifically, the permanent magnet flux and assist coil flux act to hold the plunger in a retracted position. When a trip condition occurs, the trip coil flux and the phase conductor flux act to assist movement of the plunger to the extended, tripped position.
The assist coil is energized only when the current through the phase conductor increases to a level such that the flux generated by the external phase conductor begins to overcome the permanent magnet flux. Without the assist coil, the spring force would move the plunger to the extended position. The energized assist coil creates a flux which aids the permanent magnet in retaining the plunger in the retracted position until the trip coil is energized by an associated electronic trip circuit signal. The two coils required by this arrangement increase the cost of the circuit breaker. In addition, more space may be required to mount the additional coil.
There remains a need for a device which overcomes the effects of magnetic flux in electronic circuits. More particularly, there remains a need for a device which will reduce the effects of flux created by the conductor or associated current transformers mounted in close proximity to a flux shunt trip device in a circuit breaker. There remains a further need for a device which can be used with the single trip coil which already exists in the device.